1. Field of the Invention
The present invention relates to an ink-jet head to be employed in an ink-jet printing apparatus which ejects a printing liquid (ink and so forth) through ejection openings (orifices) as flying liquid droplets and thus performs printing by depositing the liquid droplets on a printing medium, and to an ink-jet printing apparatus employing the ink-jet head.
It should be noted that, throughout the description and claims, the term "record" refers not only to recording characters but also to printing drawings, patterns and so forth.
2. Description of the Related Art
A typical one of conventional ink-jet heads has been disclosed in Japanese Patent Application Laid-open No. 132253/1980. The disclosed ink-jet head is constructed by fitting, onto a substrate formed with electrothermal transducers by way of film formation technology, an upper plate having ink-ejection openings, ink passages to be heat acting portions of the electrothermal transducers and a common liquid chamber for supplying ink to the ink passages. Also, a driver circuit for driving the electrothermal transducers arranged on the substrate is built in the substrate. Furthermore, a wiring substrate for wiring the driver circuit is provided. The substrate and the wiring substrate are located adjacently to each other on a heat radiating member.
With reference to FIGS. 5 and 6, the conventional ink-jet head will be further discussed hereinafter.
FIG. 5 is a schematic perspective view of the ink-jet head having the construction described above, and FIG. 6 is a schematic sectional view taken along line A--A of FIG. 5.
In FIG. 6, reference numeral 112 denotes a substrate, on which a plurality of electrothermal transducers are arranged; 113 denotes an upper plate which integrally incorporates a plurality of ink ejection openings 101, nozzle portions 102 serving as ink passages communicated with the ink ejection openings 101, a common liquid chamber 104 for supplying ink to respective nozzle portions 102, an ink supply cylinder 116 having an opening portion 105 in the upper surface of the common liquid chamber 104, and a liquid chamber frame portion 106 serving as a contacting portion to the substrate 112; 119 denotes the upper flat portion of the common liquid chamber 104; and 111 denotes a metal support member for constructing the ink-jet head by assembling various parts thereto.
Furthermore, as shown in FIG. 5, a wiring substrate 110 is arranged on the metal support member 111, and connected to the substrate 112 via bonding wires 109. The bonding wire 109 is adapted to electrically connect a pad 107 on the substrate 112 to a pad 108 on the wiring substrate 110.
In the afore-mentioned construction, discussing a flow of ink supply upon ink ejection, the ink flowing out of a tank (not shown) for storing the ink passes through various connection tubes and so forth to reach the ink supply cylinder 116 of the upper plate 113 and is temporarily accumulated in the common liquid chamber 104 via the opening 105 on the upper surface of the liquid chamber. The ink also flows to the respective nozzle portions 102 and is retained at the end of the ejection openings 101 by surface tension and so forth. Under this condition, when ejection signals are applied to the electrothermal transducers on the substrate 112, bubbles are generated in the ink within the corresponding nozzles 102, and ink droplets are ejected through the ink ejection openings 101.
Accordingly, when ejection is initiated, a required quantity of the ink has to be sequentially supplied to the nozzle portion from the common liquid chamber 104.
At this time, when bubbles 300 are present in the common liquid chamber 104 for some reason, ink supply to the nozzle portion is temporarily interrupted to make ink ejection unstable, and thus causing degradation of printing quality. Furthermore, when such condition is continued, ink ejection failure can be caused to thus make printing impossible.
The bubbles 300 may be generated either during printing or before initiation of printing. When the bubbles are generated during printing, a user may perform a recovery operation by means of capping and so forth to remove the bubbles from the liquid chamber. On the other hand, when bubbles have already been generated before initiation of printing, the recovery operation is automatically performed before starting ink ejection to remove the bubbles 300 from the liquid chamber.
Furthermore, concerning the ink-jet head maintained in the unused condition (a condition without performing printing) for a long period, the bubbles 300 may be grown by permeation of gas from the outside and/or penetration of air through the ejection openings and is retained (299) in the upper flat portion 119 of the common liquid chamber 104. Such bubbles residing in the common liquid chamber 104 will be hereinafter referred to as residual bubbles 299. In such a case, the residual bubbles 299 retained in the upper flat portion of the common liquid chamber are not only difficult to be removed by the recovery operation due to an excessively large difference in level relative to the ejection openings, but also degradate ink wettability in the nozzle and the liquid chamber, and thus causing degradation of the printing quality upon ejection.
Therefore, when the ink-jet head is unused for a long period, a given period is stored in the apparatus per se to automatically perform recovery operation at intervals of the given periods for removal of the residual bubbles in the liquid chamber and prevention of growth of the bubbles.
In EP-A 419181 is disclosed an ink-jet head where bubbles can be removed more easily from a liquid chamber.
However, in the prior art, since the flat portion 119 is present at the upper portion in the common liquid chamber 104, the rate of growth of the residual bubbles 299 is high for the presence of a large number of portions subject to permeation of gas from the outside in the construction where the ink flow passage from the ink tank to the ink ejection openings is long or where a large number of parts are to be connected.
Also, when the liquid chamber is small, even with a relatively small residual bubble 299, a degree of degradation of wettability of the nozzle and the liquid chamber becomes significant.
Therefore, particularly in the case of longtime storage, it becomes necessary to shorten the interval of the recovery operation of the apparatus i.e., to increase the frequency of the recovery operation in order to prevent residual and growth of the bubbles 299. This clearly increases consumption of the ink (a running cost associated with exchanging of the ink tank and so forth).
On the other hand, in the prior art, since the pads 107 for connection with the bonding wire 109 are arranged at the back side of the rear end of the upper plate on the substrate, in which the electrothermal transducers are formed, the substrate is extended rearwardly in the significant length. Accordingly, it becomes necessary to provide the substrate having an area greater than that required for the common liquid chamber and the liquid chamber frame. Therefore, when a large number of substrates, each of which is formed with the electrothermal transducers by film forming technology are to be simultaneously formed of a single silicon wafer, the number of substrates to be formed of the single silicon wafer must be reduced to cause an increase in cost.